Studio Technologies 45DC - Manual (Page 18)

Issue 1, January 2015 Model 45DC User Guide
Page 18 Studio Technologies, Inc.
External Party-Line Circuit
Operation
When an interface channel’s local power
LED is not lit that interface is intended to
be connected to an external party-line
circuit. The external circuit must provide
power and termination impedances to
“create” the party-line, with the Model
45DC simply serving as a user device.
When connected to an external party-line
circuit the active LED associated with that
interface channel will light when the volt-
age on pin 2 is equal to or greater than ap-
proximately 18. If this condition is present
then normal Model 45DC operation will be
able to take place.
Auto Null
The Model 45DC contains circuitry to
automatically null the hybrid networks as-
sociated with the two party-line interfaces.
This procedure minimizes the mixing of
the audio signals being received by and
sent to each party-line interface circuit.
Normally the nulling processes are per-
formed at the time of initial Model 45DC
configuration but there’s no reason why ei-
ther or both can’t be initiated any time one
desires. The only time that an auto null
must be performed is if conditions have
changed with the party-line user devices
and wiring connected to a Model 45DC’s
party-line connector. Even a small change
to a party-line intercom circuit, such as
adding or removing a section of cable, is
often enough to require that the auto null
process be performed.
Two pushbutton switches are provided
to activate the auto null process for each
interface. To initiate auto null first requires
that an interface’s active LED be lit. When
an interface’s operating mode is set for
local power the active LED will light when
current is flowing from the internal power
supply. When the local power LED is not lit
the active LED must be lit, indicating that
sufficient DC voltage is present on pin 2
of the connected party-line circuit. Once
the active LED is lit initiating an auto
null function only requires pressing and
releasing (“tapping”) the appropriate auto
null button. An LED provides a visual indi-
cation of the status of the auto null process,
flashing when the auto null process is ac-
tive. If the auto null button is pressed when
its associated active LED is not lit the auto
null process will not start. The auto null
LED will quickly flash on and off a few times
to indicate this condition. Only one auto
null process can be active at any one time.
Nothing will happen if either auto null but-
ton is pressed when the auto null process
is active for an interface. (An interface’s
mode can be changed when the auto null
is active on the other interface.)
An auto null sequence begins in this man-
ner: If local power is being provided to the
party-line interface by the Model 45DC it
will momentarily turn off and then turn on
again. This will act as a “mic kill” signal,
ensuring that microphones associated
with the connected user devices will be
placed in their off state. Then the audio
input and output signal paths associated
with the Dante input and output channels
will mute. Next a series of tones will be
sent to the party-line interface’s audio path.
Other Model 45DC circuitry, under firmware
control, will rapidly perform adjustments to
achieve the best null possible. After the ad-
justments have been made the results are
stored in non-volatile memory. Once the
process is complete the Dante audio input
and output paths are again activated.